The majority of power conversion devices that convert direct-current power into alternating-current power employ an inverter circuit including a bridge circuit. In the bridge circuit, between a high-side reference line and a low-side reference line which are connected to the direct-current power supply, two pairs (arms) of switching elements connected in series are connected in parallel. Among the four switching elements included in the two arms, a group including a first switching element and a fourth switching element and a group including a second switching element and a third switching element operate complementarily. With this configuration, the high-side reference potential and the low-side reference potential are alternately output at adjusted times. A signal defined by these two levels passes through a subsequent filter circuit to generate alternating-current voltage having a sine wave. Hereinafter, in the present specification, this control method is referred to as a “bipolar PWM” method.
A circuit configuration in which a clamping circuit is disposed between a bridge circuit and a filter circuit has been proposed (for example, see Patent Literature (PTL) 1). With this circuit configuration, it is possible to insert a period in which output at both terminals of the bridge circuit is short-circuited by the clamping circuit. Accordingly, three levels—high-side reference potential, zero potential, and low-side reference potential—can be output to the filter circuit. In the positive region of the sine wave, a signal defined by two levels consisting of the high-side reference potential and the zero potential is output, and in the negative region of the sine wave, a signal defined by two levels consisting of the zero potential and the low-side reference potential is output. A signal defined by these three levels passes through the subsequent filter circuit to generate alternating-current voltage having a sine wave. Hereinafter, in the present specification, this control method is referred to as a “clamping” method.
Compared to the bipolar PWM method, with the clamping method, the amplitude of the power output to the filter circuit can be cut in half, making it possible to reduce loss and achieve high-efficiency power conversion.
Recent years have seen an increase in systems including power conversion devices that are connected and operate in parallel and convert direct-current power supplied from a direct-current power supply such as a solar cell, fuel cell, or battery, into alternating-current power (i.e., power conditioners).